Endgame Practice

Korchnoi (Black) resigned in the following position vs Petrosian (White). Given what I know of Korchnoi this position must be absolutely crushingly strong for White. Looking at the diagram I do not sense that. If there's a winning tactic here It is not obvious at all to me.

It is White to move. If I had this position as White vs anyone I'm confident Black would not resign. Not remotely. My goal is to find a convincing continuation here. I'm doing all analysis from this position without moving the pieces and certainly without the aid of a silicon beast. I will post my answer within a week.
Answer: My variations are now in the diagram. What this exercise showed me is that I really should try and look for a better move after finding a good one. In this case 1. Qg4 is stronger than 1. Qe4 although both moves and possibly others are winning.

Endgame Idea in a Closed Position:

A nice game by Spassky. I am not sure how to continue after 2...Qd6 though. I will analyze and post my answer within a week.
Answer:
3. Qxd6 Nxd6 4. e5 and here if Nxc4 5. bxc4 b3 6. Be4+ is a key point and White is winning. White will follow up with Nf5 pressuring the h6 pawn. Seems easy enough when its just 3 moves away but Spassky must have considered that when he sacrificed the piece.